Modification of the photosystem II acceptor side function in a D1 mutant (arginine-269-glycine) of Chlamydomonas reinhardtii

Bicarbonate anions have a strong positive influence on the electron and proton transfers in photosystem II (PS II). It has been suggested that bicarbonate binds to the non-heme iron and the Q(B) binding niche of the PS II reaction center. To investigate the potential amino acid binding environment of bicarbonate, an arginine residue (R269) of the D1 protein of PS II of Chlamydomonas reinhardtii was mutated into a glycine; our characterization of the resultant mutant (D1-R269G) shows that both the Tyr(D)(+) and Q(A)(-) Fe2+ EPR signals are substantially reduced and assembly of the tetranuclear Mn is lost (R.S. Hutchison, J. Xiong, R.T. Sayre, Govindjee, Biochim. Biophys. Acta 1277 (1996) 83-92). In order to understand the molecular implications of this mutation on the electron acceptor side of PS II, we used chlorophyll (Chl) a fluorescence as a probe of PS II structure and function, and herbicide binding as a probe for changes in the Q(B) binding niche of PS II. Chl fluorescence measurements with the heterotrophically grown D1-R269G mutant cells (or thylakoids), as compared to that of the wild type, show that: rate of electron transfer from Q(A)(-) to the plastoquinone pool, measured by flash-induced Chl a fluorescence decay kinetics, is reduced by similar to 17 fold; the minimum Chi a fluorescence yield when all Q(A)(-) is oxidized, is elevated by 2 fold; the level of stable charge separation as inferred from variable Chi fluorescence is reduced by 44%; binary oscillation pattern of variable Chl a fluorescence obtained after a series of light flashes is absent, indicative of the loss of functioning of the two-electron gate on the PS II acceptor side; 77 K PS II Chl a fluorescence emission bands (F685 and F695) are reduced by 20-30% (assuming no change in the PS I emission band). Thermoluminescence data with thylakoids show the absence of the S(2)Q(A)(-) and S(2)Q(B)(-) bands in the mutant. Herbicide C-14-terbutryn binding measurements.